Fluoroelastomers, and more specifically perfluoroelastomers, have long been used in a variety of applications that require excellent resistance to high temperature and chemical attack.
To enable crosslinking of these fluoroelastomers, it has been long known to incorporate in the fluoroelastomer polymer chain a small percentage of monomers including nitrile groups. The reactivity of such nitrile groups in the presence of a variety of curing agents, including notably aromatic tetra-amines, di(amino(thio)phenols), and organo-tin compounds, including tetraalkyl and tetraaryl compounds, is such to provide crosslinking point of supposedly triazine-type or similar structure. The crosslinking chemistry of such nitrile groups possesses high thermal stability—higher than the stability of bridging/crosslinking groups provided by alternative crosslinking chemistry.
For this reason, nitrile-cure site containing fluoroelastomers have been long recognized for their thermal stability; within this area, dual cure approaches have been already pursued in the past for further improving e.g. curing rate.
Notably, U.S. Pat. No. 5,447,993 5 Sep. 1995 discloses nitrile-containing perfluoroelastomers which are cured by a combination of a peroxide, a co-agent, which can be a diene or a triene, and a catalyst causing crosslinks to form using nitrile groups, for achieving faster cure rate, while maintaining good thermal properties in vulcanizates. According to this document, two distinct (chemical) cure processes are believed to take place, one caused by the peroxide and the polyunsaturated co-agent, and the other caused by a catalysed reaction of nitrile groups.
US 2015073111 (E.I. DUPONT DE NEMOURS) 12 Mar. 2015 is directed to certain VDF-based fluoroelastomers comprising secondary cyano-groups, which can be cured using, as curatives, diaminobisphenol AF, ammonia generators such as urea, and free radical systems such as the combination of an organic peroxide with a multifunctional coagent such as triallylisocyanurate.
EP 0606883 A (NIPPON MEKTRON) 20 Jul. 1994 pertains to a composition including a nitrile-containing fluoroelastomer and a bisaminophenol derivative curing agent.
EP 0661304 A (SOLVAY SPECIALTY POLYMERS ITALY SPA) 5 Jul. 1995 is directed to iodine-containing fluoroelastomers comprising recurring units derived from a bis-olefin, which are notably cured via peroxidic route using a combination of a peroxide and curing co-agents, among which triallylisocyanurate is specifically mentioned as preferred, and used in all exemplified embodiments.
In this area, there remains a continuous quest for curable fluoroelastomer compositions able to deliver upon curing even improved thermal and steam resistance.